Metal halide arc discharge lamps are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide arc discharge lamp includes a quartz or fused silica arc tube that is hermetically sealed within a borosilicate glass outer envelope. The arc tube, itself hermetically sealed, has tungsten electrodes sealed into opposite ends and contains a fill material including mercury, metal halide additives and a rare gas to facilitate starting. In some cases, particularly in high wattage lamps, the outer envelope is filled with nitrogen or another inert gas at less than atmospheric pressure. In other cases, particularly in low wattage lamps, the outer envelope is evacuated.
It has been found desirable to provide metal halide arc discharge lamps with a shroud that comprises a generally cylindrical, light-transmissive member, such as quartz, that is able to withstand high operating temperatures. The arc tube and the shroud are coaxially mounted within the lamp envelope with the arc tube located within the shroud. Preferably, the shroud is a tube that is open at both ends. In other cases, the shroud is open on one end and has a domed configuration on the other end. Shrouds for metal halide arc discharge lamps are disclosed in U.S. Pat. No. 4,499,396 issued Feb. 12, 1985 to Fohl et al.; U.S. Pat. No. 4,580,989 issued Apr. 8, 1986 to Fohl et al.; and U.S. Pat. No. 4,888,517 to Keeffe et al., issued Dec. 19, 1989. See also U.S. Pat. No. 4,281,274 issued Jul. 28, 1981 to Bechard et al. U.S. Pat. No. 5,122,706 to Parrott et al. teaches that containment can be enhanced if the OD of the arc tube is less than 3 mm from the ID of the shroud.
The shroud has several beneficial effects on lamp operation. In lamps with a gas-filled outer envelope, the shroud reduces convective heat losses from the arc tube and thereby improves the luminous output and the color temperature of the lamp. In lamps with an evacuated outer envelope, the shroud helps to equalize the temperature of the arc tube. Finally, the shroud improves the safety of the lamp by acting as a containment device in the event that the arc tube shatters; however, it has been discovered that, upon a non-passive failure of an arc tube, the shards therefrom can fracture the shroud and the shards from the shroud may be the culprit that has the capability of fracturing the outer envelope, the actual condition that the shroud was supposed to prevent.